Microbiology B - Specific Pathogen Groups

Question 1

Streptococci and Staphylococci

Streptococcus pneumoniae

What type / class of pathogen?
Method of spread & class of infection caused? Clinical Features (what diseases / presentations, etc)?

Answer 1

• Gram Positive cocci
• Facultative anaerobes
• Encapsulated, alpha haemolytic acid.
• Methods of spread:
  
  • Streptococcus pneumoniae is a common nasopharyngeal commensal.
  • Spread by droplet.
  • Exogenous (respiratory droplet spread from carrier infecting another host) or endogenous (carrier develops impaired resistance to organism) infections may occur.

Question 2

Streptococci and Staphylococci

Streptococcus pyogenes

What type / class of pathogen?
Method of spread & class of infection caused?

Answer 2

• Gram Positive cocci
• Facultative anaerobes
• Group A, beta-haemolytic Streptococci
• Methods of Spread:
  
  • S. pyogenes is one of the most common human bacterial pathogens.
  • Invades apparently intact skin and mucous membranes, rapidly and progressively.
  • Resides on skin/mucous membranes (especially nasopharynx) of infected patients and healthy carriers.
  • Respiratory droplet or skin contact spread.

Question 3

Streptococci and Staphylococci

Streptococcus pyogenes

Clinical Features (what diseases / presentations, etc)?

Answer 3

• Clinical features:
  
  • Acute pharyngitis / tonsillitis:
    
    • S. pyogenes is the most common cause of sore throats.
    • Induces purulent inflammation in the posterior oropharynx and tonsils and may lead to scarlet fever.
  • Impetigo:
    
    • A highly contagious, localised, superficial spreading, crusty skin lesion common in children.
  • Erysipelas:
    
    • A superficial skin infection with fiery red advancing erythema on face/lower limbs.
  • Necrotising Fasciitis:
    
    • Deep infection involving fascia and subcutaneous tissues, which may progress to pyogenic myositis / myconecrosis.
    • Spreads rapidly, causing bacteraemia and sepsis.
    • Presents with fever and severe pain disproportionate to examination findings.
    • Skin signs include slight erythema, tenderness and swelling.
    • In later stages skin becomes discoloured and haemorrhagic blisters and skin necrosis develop.
    • Toxic shock may occur and mortality is high.
    • Requires rapid surgical debridement.
  • Puerperal sepsis:
    
    • Endometrial infection postpartum, presenting with purulent vaginal discharge, high fever and systemic upset.
  • Streptococcal toxic shock-like syndrome:
    
    • Due to streptococcal exotoxins commonly associated with necrotising fasciitis and packing of cavities (e.g. tampons, wounds)
    • Can be caused by staphylococcal species also.
    • Causes septic shock / multiorgan failure.
  • Scarlet Fever:
    
    • Also due to Streptococcal exotoxins.
    • Diffuse erythematous sunburn-like rash.
    • Initially white tongue, then strawberry red.
    • Complete recovery is usual with antibiotics.
  • Acute rheumatic fever:
    
    • Autoimmune disease, occurs 2-3 weeks following pharyngitis.
    • Presents with fever, rash, endocarditis and arthritis.
  • Acute glomerulonephritis:
    
    • Rare autoimmune mediated.
    • Can follow any streptococcal infection.

Question 4

Streptococci and Staphylococci

Alpha-haemolytic Streptococci

What type / class of pathogen?
Who is most at risk (esp. of endocarditis)?

** Key point - relevance in endocarditis.

Answer 4

• Gram Positive cocci
• Facultative anaerobes
• “Viridans Streptococci”
• A nasopharyngeal commensal.
• Most common cause of bacterial endocarditis.
• Risk of introduction to bloodstream during interventional procedures.
• People at risk of infective endocarditis include those with:
  
  • Valve replacements.
  • Valvular heat disease
  • Structural congenital heart disease
  • Hypertrophic cardiomyopathy
  • Previous infectious endocarditis
• In the past, antibiotic prophylaxis was given routinely to those at risk of endocarditis undergoing interventional procedures. This practice has been recently reviewed. NICE Guidelines (Mar 2008) now recommend antibiotic prophylaxis is not routinely given in dental procedures, GI, genitourinary, gynaecological or respiratory tract intervention.

Question 5

Streptococci and Staphylococci

Staphylococcus aureus

What type / class of pathogen?
Who is most at risk?

Answer 5

• Gram positive coccus
• One of the most common causes of bacterial infection.
• Methods of Spread:
  
  • Part of normal skin / mucous membrane flora.
  • Survives for along periods on inanimate objects.
  • Transmission via direct contact, contaminated objects / food.
  • Infects via both exogenous and endogenous route.
  • S. aureus produces toxins that can cause disease in absence of invasive infection.

Question 6

Streptococci and Staphylococci

Streptococcus pneumoniae

Clinical Features (what diseases / presentations, etc)?

Answer 6

• Clinical Features:
  
  • Most common cause of pneumonia and otitis media.
  • An important cause of meningitis, resulting in high mortality.
  • Frequent agent in bacteraemia and sepsis in the absence of an obvious focus of infection.
  • Also responsible for sinusitis, osteomyelitis, septic arthritis, endocarditis, peritonitis, cellulitis and brain abscesses.

Question 7

Streptococci and Staphylococci

Staphylococcus aureus

Clinical Features (what diseases / presentations, etc)?

Answer 7

• Clinical Features:
  
  • Common cause of nosocomial infections.
  • In immunocompetent patients usually remains localised at the poral of entry by host defences.
  • Skin infections include folliculitis, abscess, wound infection, cellulitis and impetigo (although impetigo is more commonly associated with Streptococcus pyogenes).
  • Osteomyelitis and septic arthritis.
  • Endocarditis occurs in IVDUs due to contaminated needles.
  • Necrotising pneumonia.

Question 8

Streptococci and Staphylococci

Staphylococcus aureus

Toxinoses

Answer 8

• Scalded skin syndrome:
  
  • Epithelial desquamation resulting from exfoliative toxin production.
• Toxic Shock Syndrome:
  
  • Caused by S. aureus exotoxins.
  • Associated with Staphylococcus wound infection, tampon use, cellulitis, osteomyelitis, postpartum.
  • Presents as high fever, confusion, vomiting, diarrhoea resulting in high mortality.
  • Septic shock and multiorgan failure may develop, resulting in high mortality.
• Staphylococcal gastroenteritis:
  
  • Develops following ingestion of food containing enterotoxin-producing S. aureus.
  • Incubation period <6hrs.
  • Clinical features are enterotoxin-mediated, rather than invasive infection.

Question 9

Streptococci and Staphylococci

MRSA

Answer 9

• Methicillin-resistant.
• A nasopharyngeal/ skin commensal.
• Resistant to multiple antibiotics.
• Associated with worse outcomes :
  
  • Longer hospital stays.
  • Longer ITU stays.
  • Higher mortality rates.

Question 10

Streptococci and Staphylococci

Staphylococcus epidermidis

General info, Methods of Spread, Clinical Features.

Answer 10

• Coagulase negative Staphylococci
• Facultative anaerobe.
• Frequently a contaminant of blood cultures.
• Methods of Spread:
  
  • Part of normal skin flora.
  • Transmitted via direct contact.
  • Opportunistic pathogen, usually hospital acquired.
• Clinical Features:
  
  • Prosthetic implant infections - catheter & heart valves.
  • Produces and extracellular polysaccharide ‘slime’ that facilitates adherence to bioprosthetic material.

Question 11

Tuberculosis

General Info

Answer 11

• Mycoplasma tuberculosis
• Gram-negative, acid fast bacilli.
• Obligate aerobe.
• 95% of all cases of TB worldwide occur in developing countries.

Question 12

Tuberculosis

Transmission

** Key - Primary means of transmission

Answer 12

• Large numbers of organisms are released by coughing.
• Transmission therefore primarily by respiratory droplet (aerosol).
  
  • Frequently the first site of infection is pulmonary.

Question 13

Tuberculosis

Establishment in Host

Stages of establishment (5, including reactivation). Risk of reactivation, including risk factors.

Answer 13

• Initial Phase
• Tubercle Formation
• Dormant
• Spread
• Reactivation

Question 14

Tuberculosis

Resistance to M. tuberculosis

Answer 14

• M. tuberculosis resistance relies on cell-mediated immune response from CD4+ T cells. Although antibodies may be generated they do not convey resistance because M. tuberculosis is intracellular.